The present invention generally relates to display (LCD) systems, and more particularly, to drive schemes for display systems.
Displays are commonly used to convey information in a variety of fields. Computers, signs, telephones, televisions, kitchen appliances, vehicle cockpits and innumerable other devices use electronic displays. Various applications require different kinds of displays, and display technology continuously advances to satisfy needs and improve performance of displays in old and new applications alike.
For example, bistable displays, such as cholesteric displays, operate in many environments and applications. Bistable pixels typically exhibit at least two stable states, one generally reflective and the other generally transmissive, such that the pixel tends to retain the selected state after the voltage across the pixel is removed. By selectively placing the various pixels in one state or another, an image may be formed on the display that is retained when the voltage across the liquid crystal material is removed. In some bistable displays, pixels may have only two states, whereas other displays provide shades of black, white, gray, colors, and the like, depending on the needs of the display system, using areas of dark or bright domains within the pixel.
A drive circuit typically generates the voltages across the pixels to change the display. In many conventional displays, the drive circuit applies signals to a series of row and column electrodes that define a matrix of pixels. The signals are provided according to a selected drive scheme to determine how the signals are applied to the various electrodes to achieve the desired image on the display. A standard drive scheme is illustrated in Catchpole et al., U.S. Pat. No. 5,644,330, issued Jul. 1, 1997, a cumulative drive scheme is illustrated in Huang, U.S. Pat. No. 6,133,895, issued Oct. 17, 2000, and a dynamic drive scheme for cholesteric LCDs is illustrated in Huang et al., U.S. Pat. No. 5,748,277, issued May 5, 1998. In another example, a progressive scanning drive scheme starts signals with a delay for each row electrode with respect to the previous row electrode. In a scanning display, each pixel in a single row is selected or deselected by a segment signal applied to the corresponding column electrode. In this manner, the segment signal may create a pixel level that is either xe2x80x9cselectxe2x80x9d or xe2x80x9cnon-selectxe2x80x9d, that is, transmissive or reflective. The states of pixels in other rows are essentially not influenced by the segment signals.
Conventional drive schemes for cholesteric displays, however, require complicated drive circuitry. For example, a drive circuit configured to implement a conventional drive scheme may provide a common signal having four different phases (preparation, selection, evolution, and final) and a segment signal that selects the state of the relevant pixel. Each phase of the common signal normally includes two or more voltage levels, and the segment signal normally requires at least two voltage levels. Thus, to implement the common and segment signals, the drive circuit may provide ten or more different voltage levels to the electrodes.
In addition, the appropriate voltage levels to achieve desired pixel states might not remain uniform over time and conditions. For example, the temperature of the liquid crystal material may affect its optical characteristics. To compensate for temperature variations, the signal voltage levels may need to be adjusted as a function of temperature for improved display performance. Such adjustments are typically made by adjusting or tuning one or more of the voltage levels of the common signal, for example. Conventional systems might use one or more variable voltage supplies. Such supplies, however, often add cost, complexity, and power consumption to the drive circuit.
A display system according to various aspects of the present invention includes methods and apparatus for driving a cholesteric display system using pulse width modulation (PWM) to control the RMS voltage applied across the pixels of the display. By using PWM to control the overall voltage, fewer voltage levels may be used during one or more phases. Further, PWM may affect the RMS voltage applied across the liquid crystal material to compensate for display variations such as optical effects due to temperature changes or to achieve other effects upon the liquid crystal.